Process for manufacturing a micro-structured optical fibre

ABSTRACT

In a process for manufacturing a micro-structured optical fibre, an intermediate preform is made by forming a sol; pouring the sol in a cylindrical mould including a set of structural generating elements defining internal structural elements of the intermediate preform; transforming the sol into a gel so as to obtain a cylindrical gel body defining the intermediate preform; and removing the cylindrical intermediate preform from the mould.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/EP01/15261, filed Dec. 21, 2001, the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for manufacturing a micro-structuredoptical fibre and to a method for producing a structured intermediatepreform to be used in such a process.

2. Description of the Related Art

Optical fibres are used for transmitting light from one place toanother. Normally, optical fibres are made of more than one material. Afirst material is used to form a central light-carrying part of thefibre known as the core, while a second material surrounds the firstmaterial and forms a part of the fibre known as the cladding. Light canbecome trapped within the core by total internal reflection at thecore/cladding interface.

These conventional fibres are typically produced by well-known vapourdeposition techniques, such as MCVD (Modified Chemical VaporDeposition), OVD (Outside Vapor Deposition) and VAD (Vapor-phase AxialDeposition).

A more recent type of optical fibre waveguide, having a significantlydifferent structure from that of conventional optical fibres, is themicro-structured fibre (also known as “photonic crystal fibre” or “holeyfiber”). A micro-structured optical fibre is a fibre made of a samehomogeneous material (typically silica), having inside a micro-structure(i.e. a structure on the scale of the optical wavelength) defined bymicro-structural elements extending longitudinally along the fibre andhaving a predetermined distribution. As a micro-structural element it ispossible to identify a micro-hole or a filiform element of a differentmaterial than the bulk.

The most common type of micro-structured optical fibre has a claddingregion showing a plurality of equally-spaced tiny holes, surrounding ahomogeneous and uniform central (core) region. A fibre of this type isdescribed, for example, in international patent application WO 99/00685.In a different embodiment, the central region of the fibre may have acentral hole, as described, for example, in international patentapplication WO 00/60388.

These two types of fibres convey light in the core according todifferent optical phenomena.

In the absence of a central hole, propagation of light in the claddingregion is forbidden due to the presence of a lowering of the averagerefractive index with respect to the core region. Such a structure formsa low-loss all-silica optical waveguide that, for appropriareparameters, remains monomode for all wavelengths within the transmissionwindow of the silica. The waveguiding mechanism in that case is closelyrelated to that in conventional optical fibres and is a form of totalinternal reflection between two materials (air and silica) havingdifferent refractive indexes.

In the presence of a central hole, propagation in the cladding region isforbidden due the presence of a “photonic band-gap”. The “photonicband-gap” phenomenon, which is analogous to the “electronic band-gap”known in solid-state physics, avoids light of certain frequencies topropagate in the zone occupied by the array of holes, this light beingtherefore confined in the core region. Propagation of light in fibresshowing a photonic band gap is described, for example, in J. C. Knight,J. Broeng, T. A. Birks and P. St. J. Russell, “Photonic Band GapGuidance in Optical Fibres”, Science 282 1476 (1998)).

Optical characteristics of the above-described micro-structured fibresdepend on the number of holes, the holes diameter, the reciprocaldistance between adjacent holes and the hole geometrical pattern. Sinceeach of these parameters can broadly vary, fibres of very differentcharacteristics can be designed.

Micro-structured optical fibres are typically manufactured by theso-called “stack-and-draw” method, wherein an array of silica rodsand/or tubes are stacked in a close-packed arrangement to form a prefom,that can be drawn into fibre using a conventional tower setup.

In U.S. Pat. No. 5,802,236A, for example, a core element (e.g., a silicarod) and a multiplicity of capillary tubes (e.g., silica tubes) areprovided, and the capillary tubes are arranged as a bundle, with thecore element typically in the center of the bundle. The bundle is heldtogether by one or more overclad tubes that are collapsed onto thebundle. The fibre is then drawn from the thus prepared preform.

A different stack-and-draw method is described in the above-cited patentapplication WO 99/00685, and comprises:

-   -   producing a cylindrical rod of fused silica;    -   drilling a hole centrally along the length of the rod;    -   milling the outside of the rod to obtain six flats so as to        confer to the rod a hexagonal cross section;    -   drawing the rod into a cane by using a fibre drawing tower;    -   cutting the cane into the required length;    -   stacking a plurality of such canes to form a hexagonal array of        canes, the cane at the centre (that will define the core of the        fibre) having no hole drilled through the center; and    -   drawing the stack of canes into the final fibre using the fibre        drawing tower.

The Applicant has noted that the stack-and-draw manufacturing method hasseveral drawbacks.

The awkwardness of assembling hundreds of very thin canes (defined byrods or tubes), as well as the possible presence of interstitialcavities when stacking and drawing cylindrical canes, may affectdramatically the fibre attenuation by introducing impurities, undesiredinterfaces and inducing a reshaping or deformation of the startingholes. Other problems of the stack-and-draw method may be represented bythe low purity of the tubes and/or rods materials and by thedifficulties in producing tubes and/or rods of the required shapes (inparticular, in the case of hexagonal bodies) and dimensions and inobtaining the required pattern of holes (due for example to thedifficulty in realizing geometries different from triangular whenpositioning rods and tubes in close-packed arrangement). Moreover, therelatively low productivity and high cost make this method not muchsuitable for industrial production.

A further drawback of the stack-and-draw method, described for examplein US 2001/0029756, is that the outer air holes of the fibre aretypically closed or are much smaller than the inner air holes. Hence,during the drawing of an optical fibre from the preform, relativelylarge inner air holes are transformed to an oval shape since the outerglass tubes are melted faster than the inner glass tubes due to thedifference in the heat conductivity between the inner portion and theouter portion of the optical fibre preform. This type of distortion inthe air holes makes the continuous mass production of holey opticalfibres very difficult.

To solve the above problem, US 2001/0029756 proposes, instead ofarranging the plurality of glass tubes as in the conventionalstack-and-draw method, to vertically arrange the plurality of glasstubes in a gel to prevent the distortion of air holes during the drawingstep of the optical fibre. In more detail, US 2001/0029756 proposes thefollowing method for fabricating a holey optical fibre. A sol is firstformed by mixing a starting material, deionized water and an additive.The sol is filled into a circular frame and gelled, and a preform rod isinserted into the center of the resulting gel. Meanwhile, a plurality ofglass tubes is vertically arranged around the preform rod in the gel.Then, the gel is removed from the circular frame and dried. The dry gelis glassified through a heat application during the sintering process.Thereafter, the holey optical fibre is drawn from the holey opticalfibre preform resulting from the sintering process by supplying gas intothe ends of the air holes in the holey optical fibre preform whileheating the other ends of the air holes, thereby preventing distorsionof air holes.

The Applicant observes that the method for fabricating a holey opticalfibre proposed by US 2001/0029756 has the drawback that the holes andthe core dimensions in the final fibre are limited by the inner andouter diameter of the tubes and rod employed in the assembly, which isalso one of the limits of the conventional stack-and-draw methodpreviously described.

Accordingly, the Applicant has tackled the problem of providing aprocess for manufacturing a micro-structured optical fibre thatovercomes the above-mentioned problems of the known techniques.

SUMMARY OF THE INVENTION

The Applicant has found that, by transforming a suitable sol into a gelin a mould containing a predetermined arrangement of removablestructure-generating elements (defined by rod-like or tubular members),and then removing the structure-generating elements, it is possible toform a structured gel preform having a predetermined internal pattern ofholes, which is suitable to be transformed into a glass preform forproducing an optical fibre. One or more of the structure-generatingelements, for example a central structure-generating element, may bedesigned to remain in the preform for modifying the optical ormechanical property thereof. The resulting structured gel preform canthen be dried and sintered to obtain a structured glass preform, whichcan successively be drawn into an optical fibre having the desiredmicro-structure.

According to a first aspect, the present invention relates to a methodfor forming an intermediate preform for manufacturing an optical fibre,comprising:

-   -   forming a sol containing a glass precursor;    -   pouring the sol in a cylindrical mould including a set of        structural generating elements apt to define internal structural        elements of the intermediate: preform;    -   transforming the sol into a gel so as to obtain a cylindrical        gel body defining the intermediate preform; and    -   removing the cylindrical intermediate preform from the mould.

Preferably, after transforming the sol into a gel, the method comprisesremoving at least one of the structural generating elements for formingat least a hole inside the intermediate preform. More preferably,removing at least one of the structural generating elements-comprisesremoving a plurality of the structural generating elements for forming apredetermined pattern of holes inside the intermediate preform.

Transforming the sol into a gel preferably comprises aging the sol for apredetermined time, while forming a sol preferably comprises mixing atleast one glass precursor and water.

The structural generating elements may be rod-like or tubular members.The set of structural generating elements preferably comprises aplurality of structural generating elements arranged around a centralaxis of the mould. Moreover, the set of structural generating elementsmay comprise one central structural generating element coaxial to thecentral axis.

According to a further aspect, the present invention relates to anintermediate preform as obtainable by the method previously described.

According to a further aspect, the present invention relates to aprocess for manufacturing a micro-structured optical fibre, comprising:

-   -   forming an intermediate preform according to the method        previously described;    -   drying the intermediate preform, for example by supercritical        drying.    -   sintering the dried intermediate preform to obtain a glass        preform; and    -   structurally modifying the glass preform to obtain the        micro-structured optical fibre.

Preferably, forming an intermediate preform comprises forming at least ahole inside the intermediate preform, and the process further comprises,after drying the intermediate preform and before sintering the driedintermediate preform, or after sintering the dried intermediate preformand before structurally modifying the cylindrical glass preform,inserting at least a micro-structural generating element into the atleast a hole.

Structurally modifying the glass preform may comprise stretching theglass preform to obtain a core rod and may comprise applying a tubularglass member externally to the core rod to obtain a final preform.

Applying a tubular glass member externally to the core rod may compriselowering the air pressure between the tubular glass member and the corerod.

Preferably, the core rod has at least a hole and applying a tubularglass member externally to the core rod comprises flowing ahydrogen-free gas into the at least a hole and controlling the pressureof the hydrogen-free gas.

Structurally modifying the glass preform may alternatively comprisedepositing glass soot onto the core rod to obtain a final preform andsintering the final prefom.

Structurally modifying the glass preform preferably comprises drawingthe final preform to obtain the micro-structured optical fibre.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details may be found in the following description, which refersto the appended figures listed here:

FIGS. 1 a, 1 b and 1 c show three different micro-structured opticalfibres;

FIG. 2 is a block representation of an assembly for manufacturing amicro-structured optical fibres according to the present invention;

FIG. 3 shows a mould that is part of the assembly of the presentinvention;

FIG. 4 shows a rod-in-tube assembly that is part of the assembly of thepresent invention;

FIG. 5 illustrates a drawing tower that is part of the assembly of thepresent invention;

FIGS. 6 a to 6 m shows schematically the different steps of the processof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 a to 1 c illustrate, as an example, three differentmicro-structured optical fibres, indicated with 1, 1′, 1″, respectively,which can be obtained by a process according to the present invention,as described in the following.

Fibre 1 (FIG. 1 a) has a central axis 2, a central region 3 coaxial toaxis 2 and an annular region 4 surrounding the central region 3. Annularregion 4 has a plurality of holes 5, which are preferably arrangedsymmetrically about axis 2 and have typically the same dimension. Holes5 may also have different dimensions, for example as illustrated in U.S.Pat. No. 5,802,236, wherein the holes of an inner crown (surrounding thecore region) have higher dimensions than the more external holes.

Central region 3 is preferably made, in this embodiment, of the samematerial as the annular region 4; in particular, central and annularregions 3, 4 are, in this case, different portions of a same homogeneousbody (apart from the discontinuity represented by holes 5). Centralregion 3 is void of holes, thus defining a “central defect” in the holeyfibre. Holes 5 may contain air or a different gas, or may be filled witha liquid or with material that is different from that of the hostingglass body. If the holes 5 are filled with a different material, thismaterial will typically have a different refractive index than thesurrounding material.

Fibre 1′ (FIG. 1 b) differs from fibre 1 in that the central region 3has a central hole 6 coaxial to axis 2, while fibre 1″ (FIG. 1 c)differs from fibre 1 in that the central region 3 comprises a centralmicro-structural element 7 of a different material than the material ofannular region 4.

Fibres 1, 1′ and 1″ thus have a plurality of micro-structural elements,which can be defined either by longitudinal holes or by longitudinalportions of a different material than the hosting glass.

The parameters characterizing the above-described micro-structuredfibres are the diameter d or d₁ of the holes 5, the diameter d₂ of thecentral hole 6 or of the central structural element 7, the spacing(pitch) Λ between two adjacent holes 5 and the external diameter D ofthe fibre. The fibre properties depend, at a chosen light wavelength λ,on the ratios d/Λ and Λ/λ. Typically, the quantities d and Λ are in themicron scale and D is, for a standard fibre, 125 μm. The ratio d/Λ ispreferably comprised between 0.1 and 0.5 and the ratio Λ/λ is preferablycomprised between 0.5 and 10, while the ratio d/D is preferablycomprised between 0.004 and 0.08 (a typical value being 1/125).

If the diameter d of holes 5 is a sufficiently small fraction of thepitch Λ, the core 3 of the fibre 1 guides light in a single mode.

An assembly apt to manufacture micro-structured fibres of the typespreviously described is schematically depicted in the blockrepresentation of FIG. 2 and is here indicated with 10. Assembly 10comprises a mould 20 for producing a gel core preform from a sol, afurnace 30 for sintering the gel core preform after drying thereof, thusobtaining an intermediate glass core preform, a stretching device 40 forstretching the intermediate glass core preform into a core rod, arod-in-tube assembly 50 to apply an external cladding to the core rodthus obtaining a final preform, and a drawing tower 70 for drawing anoptical fibre from the final preform. A dashed line shows the sequentialorder of operation of the different components of assembly 10.

With reference to FIG. 3, mould 20 comprises a cylindrical container 21wherein the sol will be formed into the gel core preform, and a set ofspaced structure-generating elements 22, 23, defined by wires, rods ortubes crossing longitudinally the container 21, for defining theinternal structure of the gel core preform. This internal structure ofthe preform will correspond to the internal microstructure of the finalfibre, and the structure-generating elements will therefore referred toas microstructure-generating elements.

Container 21 comprises a cylindrical lateral wall 24 having a centralaxis 29, and a first and a second cover 25 and 26—lower and upper,respectively—that fit with the extremities of the lateral wall 24 andcan be coupled therewith by appropriated means (not shown), for exampletie rods and nuts, screw caps or flanged connections. Sealing members 27may be interposed between the lateral wall 24 and the covers 25, 26, inorder to avoid passage of fluids from the internal of the container 21to the external, or vice versa. The choice of the diameter of container21 is based on practical considerations of easy handling and processing.

The lateral wall 24 may be a tubular member made of glass, plastic, ormetal. Covers 25, 26 may be disk-like members made of PTFE. Upper cover25 preferably has a central passing hole 25 a and a plurality ofsurrounding passing holes 25 b, preferably of smaller dimensions thancentral hole 25 a. Lower cover 26 may have a central recess 26 a and aplurality of surrounding recesses 25 b arranged as holes 25 b. In placeof lower cover 26 there may be a base wall integral with lateral wall,so as to form a one-piece cup-shaped container. Covers 25, 26 shall becoupled to lateral wall 24 so that the holes of cover 25 are alignedwith recesses of cover 26. Means may be provided for easily allowingthis alignment, like reference signs or coupling by pins. Preferably,upper cover 25 is relatively thick, so as to provide a guiding functionfor the microstructure-generating elements 22, 23.

The set of microstructure-generating elements 22, 23 preferablycomprises a central microstructure-generating element 22 coaxial to axis29 and apt to pass through central hole 25 a and engage recess 26 a, anda plurality of surrounding microstructure-generating element 23, whichare preferably of smaller cross-section than central element 22 and areapt to pass through the plurality of surrounding holes 25 b and engagerecesses 26 b. Central microstructure-generating element 22 will beabsent if a fibre like fibre 1 of FIG. 1 a has to be producedMicrostructure-generating elements 22, 23 should have dimensions andrigidity that allow easy handling and easy mould assembly. Thesurrounding microstructure-generating elements 23 may be identicalcylindrical members or may be of different sizes and of differentmaterials.

The materials of microstructure-generating elements 22, 23 should besuch that do not corrode in the sol polymerization process and, forthose elements designed to be extracted from container 21 as hereinbelow described, that do not damage in the extraction operation.Preferably, the microstructure-generating elements 22, 23 are made ofmetal, plastic, rubber or glass. The material may also be chosen inaccordance with the size of the holes that have to be formed; the choiceof the material will moreover determine the technique for removing theelements 22, 23 from the container 21. For example, for holes ofrelatively small dimensions (up to few mm), elements 22, 23 arepreferably rigid elements coated with a non-adhesive substance likePTFE, which can be pulled out by applying some kind of mechanical load,at room temperature. Differently, in the case of holes of relativelylarge cross-section (several mm or more), elements 22, 23 are preferablymade of an elastomeric material, such as rubber, and they can be stillbe pulled out by applying a mechanical load, at room temperature; inthis case, because under tensile stress the diameter of a rubber tube orrod is reduced by a factor related to the Poisson ratio of the material,the risk of damage of the internal surface is limited.

Alternatively (and less preferably) to mechanical extraction techniques,chemical removal techniques can be used. Accordingly, elements 22, 23may be made of a dissolvable, soluble or low melting point substance.For example, elements may be made of a polymer or paraffin that can beremoved by use of a solvent or by melting with a bland thermaltreatment. Also elements 22, 23 made of a burnable material (such as agraphite, a polymer, etc.) can be used, so that removal can beaccomplished by combustion during the sintering step. In the last casethe material may be chosen so as to be resistant to supercritical dryingconditions like those described below.

Microstructure-generating elements 22, 23, if required, may be holdstraight and in tension by some appropriated means: for example one endthereof can be fixed, and the other one clamped and tensioned with aweight or some mechanical device.

One or more elements of the set of elements 22, 23 may be designed toremain embedded in the preform so as to become a structural element ofthe preform and, then, a micro-structural element of the fibre. Thefunction of these elements can be optical or mechanical. For example, toproduce fibre 1″ of FIG. 1 c, the central microstructure-generatingelements 22 is designed to remain embedded in the gel structure so as toform the central structural element 7. The elements designed to remainembedded in the preform will be made of predetermined material that isdifferent from the bulk material of the preform and is suitable tostretch when, at the end of the process for manufacturing the fibre, thefinal preform is drawn into an optical fibre. For example, the centralmicrostructure-generating elements 22 may be a glass rod containinggermanium.

The arrangement and the dimension of microstructure-generating elements23 shall be chosen so as to obtain a predetermined spatial distributionand size of holes in the fibre to be manufactured. In particular, theratio between the diameter of elements 23 and their reciprocal distanceshall correspond to the predetermined ratio d/Λ between the diameter d(or d₁) of the holes 5 in the final fibre (more in general, of themicrostructural elements in the cladding region) and their periodicityA. Differently, the ratio between the diameter of elements 23 and theinternal diameter of container 21 shall not necessarily correspond tothe ratio d/D between the diameter d of the holes and the externaldiameter D of the final fibre, and can be chosen according to exigenciesof easy handling and processing. In particular, too little dimensionsand spacing of the holes make mould assembly very difficult, whereas atoo large mould diameter complicates preform post-processing and islikely to cause scrap.

It is evident that a mould as previously described allows for totalfreedom in the design of the microstructure, and does not generate theundesired interfaces typical in the stack-and-draw method. Moreover, itcan be accurately cleaned and isolated from external ambient, so as toreduce causes of optical scattering. As a further advantage, theassembly of the mould is much easier and faster than the stacking ofcanes, and it requires much less elements.

Modifications and variations of the mould structure are possible, forexample by providing one or more microstructure-generating elements thatare integral with one of the two covers.

The process of producing a gel preform by mould 20, together with asuccessive process of transformation of the gel preform into an aerogelpreform, will be described in the following.

The sintering furnace 30 is apt to sintering the aerogel preform toobtain an intermediate glass core preform. Sintering furnace can be anyfurnace known in the art suitable to sintering a gel preform into aglass preform, in particular any furnace suitable to generate atime-variable temperature ranging at least up to 1300° in an atmosphereof helium and/or chlorine gas.

The stretching device 40 is apt to stretch the intermediate glasspreform to obtain a core rod of a predetermined diameter and can be anystretching device known in the art that is apt to stretch a glasspreform.

With reference to FIG. 4, rod-in-tube assembly 50 is apt to apply ontothe core rod, here indicated with 51, a previously realized tubular body52 made of glass, which will define the external portion of the finalpreform. Tubular body 52 contains, in a first end portion 52 a thereof,a starting rod 53 made of glass, which fits internally with the tubularbody 52 and is designed to form the “neckdown” portion of the finalpreform at the beginning of the drawing process. Preferably, rod-in-tubeassembly 50 is made to operate vertically, with the first end portion 52a and starting rod 53 defining a lower portion of the assembly.

Assembly 50 comprises a tubular body 54 defining a preform handling andsupplying member. Preform handling and supplying member 54 is preferablymade of glass and is apt to be coupled to one end of the core rod 51 bywelding. Handling and supplying member 54 allows an easy handling ofcore rod 51 for insertion of core rod 51 into tubular body 52 through asecond (upper) end portion 52 b thereof at the beginning of therod-in-tube process that will be described later. Member 54 preferablyhas the same external diameter of core rod 51. One end 54 a of handlingand supplying member 54 is designed to remain outside the tubular body52 during the rod-in-tube process and is provided on top with a lid 55.

A cup-shaped enclosure member 56, having a central hole withsubstantially the same diameter of handling and supplying member 54, isapt to be coupled to end portion 52 b of tubular body 52 and to bepassed through by handling and supplying member 54 when core rod 51 isinside tubular body 52. The external edge of the enclosure member 56 maybe bent at 90° so as to form a L-section portion that fits with thecircular corner of end portion 52 b.

Enclosure member 56 delimits, together with an annular portion of meltedglass 57 realized between tubular body 52 and core rod 51 close tostarting rod 53, a chamber 58 wherein low pressure or vacuum can becreated so as to cause the tubular body 52 to collapse onto core rod 51during the drawing process (as taught, for example, in internationalpatent application WO 99/09437). Accordingly, apparatus 50 comprises avacuum generation system 59 of a known type, suitable to pump out airfrom chamber 58 via an outlet 60 provided in the enclosure member 56, sothat air pressure inside chamber 58 can be set lower than 1 Bar.

Preferably, apparatus 50 further comprises a pressure control device 61,which is in fluid communication with the longitudinal holes of core rod51 via an air passage 62 realized in the lid 55 and via the cavityinside the handling and supplying member 54, and which comprises gaspump means for pumping into said longitudinal holes a hydrogen-free gasapt to remove the hydrogen therefrom, and pressure controlling means forcontrolling the pressure of this gas, in particular for setting apressure that is over 1 Bar.

As will be described later, having a pressure lower than 1 Bar insidechamber 58 allows tubular body 52 to collapse onto core rod 51 duringthe fibre drawing process, while having a pressure over 1 Bar inside theholes of core rod 51 allows said holes to maintain substantially theoriginal shape while thinning during the fibre drawing process.Alternatively, collapsing the tubular body 52 onto core rod 51 may beperformed in a separate step before drawing, for example by using anappropriate furnace.

An alternative embodiment may be provided in the case of holes ofdifferent dimensions inside core rod 51. In this case, differentpressures may be conveniently set into the different-size holes.Accordingly, in substitution of (or in addition to) lid 55, a cover (notshown) may be provided on top of core rod 51, with a set of holes incorrespondent positions with the holes of core rod 51, for allowingfluid communication (for example via respective pipes) of pressurecontrol device 61 with said holes.

With reference to FIG. 5, drawing tower 70 comprises a plurality ofcomponents that are substantially aligned in a vertical drawingdirection (whence the term “tower”). The choice of a vertical directionin order to perform the main steps of the drawing process arises fromthe need to exploit the gravitational force so as to obtain, from thefinal glass preform 71, molten material from which a fibre 72 can bedrawn.

In detail, the tower 70 comprises a device 73 for supporting andsupplying the preform 71, a furnace 74 for performing a controlledmelting of a lower portion of the preform 71, a traction device 75 forpulling the fibre 72 from the preform 71 and a device 76 for winding thefibre 72.

The furnace 74 may be of any type designed to produce the controlledmelting of a preform. Examples of furnaces that can be used in the tower70 are described in U.S. Pat. Nos. 4,969,941 and 5,114,338. The furnace74 may be provided with a temperature sensor 77 designed to generate asignal indicative of the temperature inside the furnace 74.

Moreover, support device 73 may comprise a preform position sensor 78,providing a signal indicative of the normalized longitudinal coordinatez of the portion of the preform 71 that is melting in that instant.

A tension-monitoring device 79, designed to generate a signal indicatingthe tension of the fibre 72, may be provided underneath the furnace 74,or in any other position between furnace 74 and traction device 75. Themonitoring device 79 may be, for example, of the type described in U.S.Pat. No. 5,316,562 or of the type described in U.S. Pat. No. 5,079,433.

Drawing tower 70 may further comprise a diameter sensor 80 of a knowntype, positioned underneath monitoring device 79 in the particularembodiment here described, which is designed to generate a signalindicating the diameter of the fibre 72 without any coatings.Preferably, the diameter sensor 80 also performs the function of asurface defect detector, detecting defects in the glass of the fibre 72,such as bubbles or inclusions. The diameter sensor 80 may be, forexample, of the interferometric type. This type of sensor is designed,in particular, to generate a first signal proportional to the differencebetween the detected diameter value and a predefined diameter value, anda second signal indicating the presence of any surface defects.

A cooling device 81 may be situated underneath the furnace 74 and thediameter sensor 80 and may, for example, be of a type having a coolingcavity designed to be passed through by a flow of cooling gas. Thecooling device 81 is arranged coaxially with respect to the drawingdirection, so that the fibre 72 leaving the furnace 74 can pass itthrough. The cooling device 81 may be, for example, of the typedescribed in U.S. Pat. No. 5,314,515 or the type described in U.S. Pat.No. 4,514,205. The cooling device 81 may be provided with a temperaturesensor (not shown) designed to provide an indication of the temperaturein the cooling cavity. Since the speeds at which an optical fibre isdrawn are usually relatively high, the cooling device 81 must allowrapid cooling of the fibre 72 to a temperature suitable for thesuccessive processing steps and, in particular, suitable for the surfacecoating described below.

Preferably, tower 70 further comprises a first and a second coatingdevice 82, 83, positioned underneath the cooling device 81 in thevertical drawing direction and designed to deposit onto the fibre 72, asit passes through, a first protective coating and, respectively, asecond protective coating overlapping the first one. Each coating device82, 83 comprises, in particular, a respective application unit 82 a, 83a which is designed to apply onto fibre 72 a predefined quantity ofresin, and a respective curing unit 82 b, 83 b, for example a UV-lampoven, for curing the resin, thus providing a stable coating. The coatingdevices 82, 83 may be, for example, of the type described in U.S. Pat.No. 5,366,527 and may be more or less than two, depending on the numberof protective coatings that are to be formed on the fibre 72.

The traction device 75 is positioned underneath coating devices 82, 83and is preferably of the single pulley or double pulley type. In theillustrated embodiment, the traction device 75 comprises a singlemotor-driven pulley 84 that is designed to draw the fibre 72 in thevertical drawing direction. The traction device 75 may be provided withan angular velocity sensor 85 that is designed to generate a signalindicating the angular velocity of the pulley 84 during its operation.The speed of rotation of the pulley 84 and, therefore, the drawing speedof the fibre 72, may be varied during the drawing process.

In the case where, during the drawing process, undesired variations inthe diameter of the fibre 72 occur, the signal of the diameter sensor 80may be used to vary automatically the drawing speed of the fibre 72 soas to have again the predefined diameter value. In practice, if thediameter is reduced to below a predefined threshold, the drawing speedis decreased by an amount proportional to the reduction in diameter,while if the diameter is increased above a further predefined threshold,the drawing speed is increased by an amount proportional to the increasein diameter. Examples of the use of diameter sensor signals and surfacedefect sensors are provided by U.S. Pat. Nos. 5,551,967, 5,449,393 and5,073,179. The number and the arrangement of the diameter sensors andsurface defect sensors may be different from those indicated.

Tower 70 may also comprise a device 86 for adjusting the tension of thefibre 72 downstream the traction device 75. Device 86 is designed tocounterbalance any variations in tension of the fibre 72 between pulley84 and winding device 76. The device 86 comprises, preferably, a firstand a second pulley 86 a, 86 b that are mounted idle and in a fixedposition, and a third pulley 86 c which is free to move vertically,under the action of its own weight and the tension of the fibre 72. Inpractice, pulley 86 c is raised if there is an undesirable increase inthe tension of the fibre 72 and is lowered if there is an undesirabledecrease in the tension of the fibre 72, so as to keep the said tensionconstant. The pulley 37 c may be provided with a vertical positionsensor (not shown) that is designed to generate a signal indicating thevertical position of the pulley 86 c and therefore indicating thetension of the fibre 72.

Winding device 76 comprises a reel 87 and a motorized device 88 forsupporting and moving the reel 87. The reel 87 has an axis 87 a anddefines a cylindrical support surface for the fibre 72. Device 88 isdesigned to support the reel 87 and to set it into rotation about axis87 a.

Winding device 76 also comprises a fibre-feeding pulley 89, which may bemounted on a motorized slide (not shown) movable along an axis 89 aparallel to the reel axis 87 a, and which is designed to receive thefibre 72 from the tension-adjusting device 86 and to supply the fibre 72to the reel 87 in a direction substantially perpendicular to the axis 87a. During the process of winding of fibre 72, the controlled movement ofpulley 89 allows helical winding of fibre 72 to be performed.

As a possible alternative, pulley 89 may be mounted on a fixed supportand reel 87 may be movable in a controlled way along axis 87 a.

A further pulley 90 may be present in order to guide the fibre 72 fromthe tension-adjusting device 86 towards the pulley 89 a. Any otherpulleys (or guiding elements of another type) may be used, as required.

A control unit 91 is electrically connected to all the sensors and thedetectors present along the tower 70 and to all the components of tower70 whose operation may be controlled from the outside. Control unit 91is designed to control the various steps of the drawing process on thebasis of the values of pre-set process parameter, of the results of therefractive index measurement previously described and on the basis ofthe signals generated by the sensors and by the detectors positionedalong the tower 70. Exchange of information between unit 91 and thevarious parts of the tower 70 to which it is connected takes place bymeans of electronic interfaces (not shown) able to convert the digitalsignals generated by the said unit 91 into analogue signals (for exampleelectrical voltages) suitable for operating the individual parts, andalso to convert the analogue signals received from the sensors and thedetectors into digital signals designed to be interpreted by said unit91.

In particular, the following interfaces may be provided: a firstinterface associated to furnace 74, allowing control unit 91 both tosend a control signal to the furnace 74 so as to control itstemperature, and to receive information from the temperature sensor 77;a second interface associated to traction device 75 so as to control theangular velocity of pulley 84, and to receive information from theangular velocity sensor 85 associated with said drawing device 75; and athird interface associated to winding device 76, allowing unit 91 bothto send a control signal to motorized device 88, so as to control thespeed of rotation and of translation of reel 87, and to receive signalsfrom the angular and linear velocity sensors (not shown) associated withthe winding device 76.

A process for manufacturing a micro-structured optical fibre accordingto the present invention is herein below described with reference to theschematic representation of FIGS. 6 a to 6 m.

The process starts (FIG. 6 a) with the preparation of a liquidprecursor, in particular a raw material consisting of an inorganic sol,here indicated with 80. The sol 80 may be obtained by mixing a glassprecursor and water; in particular, the sol 80 may be obtained by achemical reaction implying metal alkoxides and water in an alcoholicsolvent. The first reaction is a hydrolysis, which induces thesubstitution of OR groups linked to silicon by silanol Si—OH groups.These chemical species may react together to form Si—O—Si (siloxane)bonds, which lead to the silica network formation. This phaseestablishes a 3D network that invades the whole volume of the container.The liquid used as solvent to perform the different chemical reactionsin these two syntheses remains within the pores of the solid network.

In a second step (FIG. 6 b), after having assembled the mould 20, theinorganic sol 80 is poured into mould 20. Sol 80 may be poured throughcentral hole 25 b before inserting central microstructure-generatingmember 22, or through an appropriate inlet (not shown) provided on uppercover 25. Structure-generating members 23 may also be inserted intocontainer 21 after pouring the sol, but before sol transformation intogel.

The appropriate temperature and pressure conditions, preferably normalenvironmental temperature and pressure, are then imposed for the timerequired for sol to harder and completely transform into a gel body(FIG. 6 c), here indicated with 81.

As a gel body is formed, the microstructure generating elements 22, 23are removed from the container 21 (FIG. 6 d) so as to form the internalstructure of said gel body. A subset of the microstructure generatingelements 22 may remain embedded in the gel body, if they are designed tobe form into microstructural portions of the fibre. In particular,central microstructure-generating element 22, if provided, may beremoved (together with elements 23) or may remain embedded in the gelbody, depending whether a fibre as in FIG. 1 b or as in FIG. 1 c has tobe produced. When a microstructure-generating element 22 or 23 isextracted, a hole of the same size and geometry is generated in the gelbody. The result of this last step is a gel core preform 82 having apredetermined internal structure.

This gel preform 82 is then extracted from the container 21 (FIG. 6 e).In particular, covers 25 and 26 are separated from lateral wall 24, andgel preform is extracted from lateral wall 24.

The gel core preform 82 is then transformed into an aerogel core preform83 (FIG. 6 f), by removing the solvent from the pores of the gelmaterial, which could cause cracking. The process of transforming gelinto aerogel can comprise ageing, solvent exchange and supercriticaldrying. Preferably, the gel is first subjected to solvent exchange, andthen drying by thermal treatment in supercritical conditions for thesolvent is performed. A process for transforming a gel into an aerogelis described, for example, in U.S. Pat. No. 5,207,814.

The aerogel core preform 83 may then be sintered in the sinteringfurnace 30, so as to obtain an intermediate glass core preform 84 (FIG.6 g) having the same ratio between holes diameter and preform externaldiameter. Sintering preferably comprises a thermal treatment for theconsolidation of the aerogel, in the presence of suitable gases, such asoxygen, chlorine and helium, for removing organic residues and water.Thermal treatment is preferably performed at a temperature that variesso as to perform oxidation of organic residuals in the aerogel,dehydration to remove water and, finally, consolidation of the aerogel.

One or more further structural elements, if required, can be inserted atthis stage in the holes previously made in the intermediate core preform(for instance a glassy core rod 85 having a different refractive index)(FIG. 6 h). Such elements, as the ones that remain in the body underformation from the beginning of the process, become structural elementsof the preform and, then, micro-structural elements of the fibre.

The function of these elements can be optical or mechanical. In theformer case the element has been inserted to vary the opticalpropagation properties of the final fibre, and it can have a higher orlower refractive index with respect to the bulk. One or more element maybe also inserted for varying the optical properties through stressinduced optical effects.

This step of inserting further structural elements may alternatively beperformed before the step of sintering the preform.

The resulting preform may then be stretched by stretching device 40 toobtain core rod 51 (FIG. 6 i). As schematically illustrated, thestretching device 40 may comprise a conventional draw furnace 41 andmotor-driven tractors 42 to draw the core rod 51 from the glass preform84. Stretching, and the successive rod-in-tube process, may be requiredto obtain a glass body with a predetermined d/D ratio. In fact, sincethe choice of the dimensions of container 21 andmicrostructure-generating elements 23 in mould 20 is based on practicalconsideration of easy handling and processing, the ratio d/D in the corepreform may be larger than that needed in the microstructured fibre.However, the dimensional ratio d/Λ is maintained. Therefore, stretchingis convenient for a further reduction of the holes diameter.

The stretched core rod 51 is then inserted in a tubular member 52 by therod-in-tube assembly 50 (FIG. 6 l), so as to obtain a structured finalpreform, having the desired d/D ratio.

Alternatively, the core rod 51 may be subjected to a vapour depositionprocess, according for example to a OVD method known in the art, fordepositing onto the core rod 51 an overcladding layer, thus obtainingthe structured final preform.

The last step of the overall process (FIG. 6 m) is the drawing of thestructured final preform, indicated with 71, by drawing tower 70 forobtaining the microstructured fibre 72.

During drawing, air pressure inside chamber 58 of the final preform canbe set lower than 1 Bar by means of vacuum generation system 59 so as toallow tubular body 52 to collapse onto core rod 51, while hydrogen-freegas at a pressure over 1 Bar is pumped into the holes of final preformby means of pressure control device 61 so that the holes can maintainsubstantially the original shape while thinning during the fibre drawingprocess.

Alternatively, collapsing the tubular body 52 onto core rod 51 may beperformed, as previously stated, in a separate step before drawing, forexample by means of an appropriate furnace. The above-described controlof air pressure inside chamber 58 can be performed in this step.

As further possible alternatives, the drawing may be performed directlyon the intermediate glass core preform 84 (obtained by sintering theaerogel core preform 83) or on the core rod 51.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed embodiment ofthe present invention without departing from the scope or spirit of theinvention.

In particular, some steps of the above-described process may be omittedor may be executed in a different order.

For example, the glass core preform obtained at the end of the sinteringstep could be directly drawn, thus omitting the steps of stretching androd-in-tube assembling. Alternatively, this glass core preform could bedirectly subjected to the rod-in-tube process without being previouslystretched.

The following examples relate to the production of different aerogelpreforms according to the process of the present invention.

EXAMPLE 1

Micro-structured fibre is manufactured as follows. First, 100 gr oftetraethylortosilicate (TEOS) are stirred for about 30 minutes at roomtemperature with 300 ml of a 0.01 N solution of hydrochloric acid. Theclear solution is then concentrated by using roto-vapor until theelimination of 140 ml of ethanol/water moisture. Then, 58 gr of fumedsilica are added to the clear solution. The resulting mixture isvigorously stirred until homogenization and then centrifuged at 1500 rpmfor 10 minutes. The pH of the colloidal solution is raised to 3.9 withammonia solution obtaining a suitable sol for the successive gelationstep. The sol is then poured into the mould 20 already assembled withthe microstructure generating elements 22, 23. The lateral wall 24 ofmould 20 is made of PTFE material. The microstructure-generatingelements 23 are stainless steel rods of 3 mm diameter and 250 mm length;the spacing between two elements is 7.5 mm (d/Λ=0.4). The pattern ofholes has a hexagonal shape and is like the one represented in FIG. 1 a.Gelation of the sol occurs in few hours. After 12-24 hours a smallshrinkage is observed and the microstructure-generating elements can bemanually extracted by pulling out them from the mould 20. The gel isthen soaked in acetone and later in ethylacetate, which is the liquidused in the further drying step. The drying is carried out insupercritical conditions. For this scope the gel is put into anautoclave of 5 liters of volume, which is then pressurized with nitrogenup to 50 Bar. The heating is then started with a rate of 100° C./hour.The pressure is increased up to 60 Bar and then it is maintainedconstant acting on a vent valve until the temperature has reached 290°C. At this point the valve is opened and the pressure is decreased witha rate of 7.5 Bar/hour. The autoclave is then cooled down to roomtemperature. The sample obtained is a structured aerogel preform freefrom defects or cracks.

EXAMPLE 2

The only difference with respect to Example 1 is represented by thediameter and the spacing of microstructure-generating elements 23: inthis case microstructure-generating elements 23 are 1 mm diameterstainless steel wires spaced of 5 mm (d/Λ=0.2). The successive steps areas in Example 1. The sample obtained is a structured aerogel preformfree from defects or cracks.

EXAMPLE 3

In this case, the pattern of holes is like the one shown in FIG. 1 b.The central microstructure-generating element 22 is a PTFE rod of 6 mmand the microstructure generating elements 23 are 1 mm diameterstainless steel wires spaced of 5 mm (d₁/Λ=0.2). The length of theelements is again 250 mm. The successive steps are as in Example 1. Thesample obtained is a structured aerogel preform free from defects orcracks.

EXAMPLE 4

The differences with respect to Example 2 are material and dimensions ofcentral microstructure-generating element 22 and spacing ofmicrostructure-generating elements 23: the centralmicrostructure-generating element 22 is a silicone rubber rod of 10 mmand the microstructure-generating elements 23 are spaced of 8 mm(d₁/Λ=0.125). When the transformation of sol into gel is over, themicrostructure-generating elements 22, 23 are again manually extracted.The sample obtained is a structured aerogel preform free from defects orcracks.

EXAMPLE 5

In this case, the pattern of holes is like the one shown in FIG. 1 c.The central microstructure-generating element 22 is a germanium dopedsilica glass core rod of 5 mm and the microstructure-generating elements23 are 1 mm diameter stainless steel wires spaced of 5 mm (d₁/Λ=0.2).The length of the elements is again 250 mm. Elements 23 are manuallyextracted while central element 22 is left inside the gel. Further stepsare performed as in Example 1. The sample obtained is a structuredaerogel preform free from defects or cracks.

EXAMPLE 6

The structured aerogel preforms obtained in the Examples from 1 to 5 aregradually heated in air up to 280° C. at heating rate of 5° C./min; from280 to 400° C. at 1°C./min; from 400 to 1200° C. at 2° C./min; thistemperature is then maintained for 6 hours and finally decreased down to25° C. The obtained transparent objects are structured core preforms.The dimensions are reduced of about 50%.

EXAMPLE 7

The structured aerogel preforms obtained in the Examples from 1 to 5 aregradually heated in air up to 280° C. at heating rate of 5° C./min; from280 to 500° C. at 1C/min under a flow of gas containing helium and 4% ofoxygen; from 500 to 1° C. at 2° C./min under a flow of chlorine andhelium; from 1100 to 1350° C. at 2° C./min under a flow of helium; thistemperature is then maintained for 6 hours and finally decreased down to25° C. The obtained transparent objects are structured core preformsfree from hydroxyl group. The dimensions are reduced of about 50%.

EXAMPLE 8

A structured aerogel core preform is manufactured as in the Example 2,by using a mould 20 having an inner diameter of 38 mm. The resultingaerogel preform is consolidated as in Example 7. After consolidation,the core preform has an external diameter of about 18 mm and holes havea diameter of about 0.5 mm. The structured core preform is welded at oneend with a tube of 18×20 mm and inserted in a big tube 19×64 mm (rod intube assembly) as in FIG. 4. The assembled structured preform is thenput inside the drawing furnace 74 and vacuum (˜0.3 bar) is generated inthe chamber 58 between core preform 51 and tubular body 52, while insidethe tubular body 52 is maintained a helium atmosphere. A microstructuraloptical fibre with holes of 1 μm is then obtained by drawing.

EXAMPLE 9

An aerogel preform produced as in the Example 4 is sintered as inExample 7. The central hole is reduced from 10 mm in the gel to 5 mm inthe glass. In that hole is inserted a germanium doped silica core rodeof 4.8 mm diameter in order to assemble a structured core preform. Sucha structured core preform is then mounted in a rod-in-tube apparatus 50as described in Example 8 and drawn to obtain a microstructured opticalfibre.

1. A process for manufacturing a micro-structured optical fibre,comprising: forming a sol containing a glass precursor; pouring the solin a mould including a set of structural generating elements defininginternal structural elements of an intermediate preform, the structuralgenerating elements being made of rubber; transforming the sol into agel so as to obtain a gel body defining the intermediate preform;removing the intermediate preform from the mould; applying a mechanicalload at room temperature to remove at least one of said structuralgenerating elements, thereby forming at least one hole in theintermediate preform; drying the intermediate preform; sintering thedried intermediate preform to obtain a glass preform; and structurallymodifying the glass preform to obtain the micro-structured opticalfibre.
 2. The process according to claim 1, wherein structurallymodifying the glass preform comprises stretching the glass preform toobtain a core rod.
 3. The process according to claim 2, whereinstructurally modifying the glass preform comprises applying a tubularglass member externally to the core rod to obtain a final preform. 4.The process according to claim 3, wherein applying a tubular glassmember externally to the core rod comprises lowering the air pressurebetween the tubular glass member and the core rod.
 5. The processaccording to claim 3, wherein the core rod has at least one hole, whichhad been formed previously in the intermediate preform, and whereinapplying a tubular glass member externally to the core rod comprisesflowing a hydrogen-free gas into said at least one hole of the core rodand controlling the pressure of said hydrogen-free gas.
 6. The processaccording to claim 3, wherein structurally modifying the glass preformcomprises drawing the final preform to obtain the micro-structuredoptical fibre.
 7. The process according to claim 2, wherein structurallymodifying the glass preform comprises depositing glass soot onto thecore rod to obtain a final preform and sintering the final preform. 8.The process according to claim 7, wherein structurally modifying theglass preform comprises drawing the final preform to obtain themicro-structured optical fibre.
 9. The process according to claim 1,wherein drying comprises supercritical drying.
 10. The process accordingto claim 1, wherein the process further comprises, after drying theintermediate preform and before sintering the dried intermediatepreform, or after sintering the dried intermediate preform and beforestructurally modifying the glass preform: inserting at least onemicro-structural generating element into said at least one hole formedin the intermediate preform.